This invention relates to the production of solid microspheres for use in controlled release of agriculturally active materials, particularly liquid materials.
There are a number of techniques available in the art for providing controlled-release formulations of such materials. Among those are procedures for encapsulating compounds in shells comprised of various polymers. The capsules may contain the agriculturally active materials in various physical forms. In some processes the capsules are formed by producing an emulsion of a liquid active ingredient (which may be in the form of the ingredient per se in liquid form or a solution of it in an appropriate solvent) in a second phase in the presence of one or more monomers or pre-polymers followed by processing steps to produce a shell of a polymer surrounding the emulsified droplets of active liquid material. In other embodiments, the active liquid ingredient may be present as an emulsion, in a solution, or as a suspension of smaller microspheres, within the capsule formed by the polymer.
One technique for producing controlled release formulations in the form of microspheres involves dispersing a liquid, water-insoluble active ingredient (which may be the ingredient per se or a solution of it in a water-immiscible solvent) in an aqueous phase containing polyvinyl alcohol, and then forming the microcapsules or microspheres by spray-drying this dispersion to remove the solvent and the water. Such a process is disclosed in general, in U.S. Pat. No. 4,244,836 of Frensch et al.
Modifications or improvements of this process are disclosed in U.S. Pat. Nos. 5,073,191 and 5,160,530 (the former being a division of the latter) of Misselbrook et al. In the process as described in those patents, a low melting agriculturally-active material, in molten form, is combined with an aqueous solution of a water-soluble film-forming polymer such as polyvinyl alcohol, at a temperature sufficient to maintain the active material in the molten state. The active material is then dispersed or emulsified in the aqueous solution and the resulting dispersion or emulsion is then spray-dried at a temperature of between approximately 50.degree. and 220.degree. C. so as to produce either microcapsules or microspheres containing the active ingredient. Additives such as plasticizers, wetting agents and anti-caking agents may be included in the aqueous phase for modification of the nature of the polymeric wall of the microcapsules or microspheres. The patent states that the product produced by this process is a dry, free-flowing powder or granule. However, problems occurred when the techniques of the Misselbrook et al. patents were applied in an attempt to produce microcapsules or microspheres containing the low-melting insecticide lambda-cyhalothrin, [.alpha.(S*),3.alpha.(Z)]-(.+-.)-cyano-(3-phenoxybenzyl)-3-(2-chloro-3,3,3 -trifluoro-1-propenyl)-2,2-dimethyl cyclopropane carboxylate. While the microsphere product produced by this technique appeared to be of good quality, it did not disperse well when mixed with water for spraying; because of the low melting point of the active ingredient, the material tended to melt, stick together, and plug up the spray nozzels.
The present invention, therefore, is directed to a process for production of microcapsules or microspheres containing an agriculturally active material (e.g. a molten low melting solid, a liquid active ingredient, an emulsion or a solution) which forms good quality microspheres and which disperses well in water for spraying.